1. Field of the Invention
The present invention relates to a method and apparatus for transmitting/receiving multimedia broadcast/multicast service (MBMS) transport blocks. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enabling a user equipment to receive the MBMS blocks more efficiently.
2. Description of the Related Art
FIG. 1 is a block diagram of a network structure of a universal mobile telecommunications system (UMTS). Referring to FIG. 1, the universal mobile telecommunications system (hereinafter, referred to as ‘UMTS’) includes a user equipment 1 (hereinafter, referred to as ‘UE’), a UMTS terrestrial radio access network 2 (hereinafter, referred to as ‘UTRAN’) and a core network 3 (hereinafter, referred to as ‘CN’). The UTRAN 2 includes at least one radio network sub-system 4 (hereinafter, referred to as ‘RNS’) and each RNS includes a radio network controller 5 (hereinafter, referred to as ‘RNC’) and at least one base station 6 (hereinafter, referred to as ‘Node B’) managed by the RNC. The Node B 6 includes at least one cell.
FIG. 2 is an architectural diagram of a radio interface protocol between a UE 1 and UTRAN 2. Referring to FIG. 2, the radio interface protocol horizontally includes a physical layer, a data link layer and a network layer and the radio interface protocol vertically includes a user plane for data information transfer and a control plane for signaling transfer. The protocol layers in FIG. 2 can be divided into L1 (first layer), L2 (second layer), and L3 (third layer) based on lower three layers of the open system interconnection (OSI) standard model widely known in the communications systems.
The physical layer as the first layer provides an information transfer service to an upper layer using physical channels. The physical layer is connected to a medium access control (MAC) layer above the physical layer via transport channels through which data are transferred between the medium access control layer and the physical layer. Data is transmitted between different physical layers, and more particularly, between the physical layer of a transmitting side and the physical layer of a receiving side via physical channels.
The medium access control (MAC) layer of the second layer provides services to a radio link control (hereinafter abbreviated RLC) layer above the MAC layer via logical channels. The RLC layer of the second layer supports reliable data transfer and is operative in segmentation and concatenation of RLC service data units (SDUs) sent down from an upper layer.
The RLC layer operates according to one of three modes; a transparent mode, an unacknowledged mode and an acknowledged mode. A major function of the RLC layer is an SDU discard function. A transmitting side RLC entity discards old RLC SDUs stored in the RLC entity in order to prevent an RLC buffer from being overloaded. The SDU discard function plays an important role in securing a quality of service (hereinafter ‘QoS) of a radio bearer service provided by the RLC layer.
A condition for the transmitting side RLC entity to discard an SDU is the use of a timer or a retransmission count limitation. In the transparent or unacknowledged mode, only the timer is used. In the acknowledged mode, both of the timer and retransmission count limitation can be used.
When an RLC layer operates in transparent mode, no header information is added to an RLC SDU transferred from an upper layer and data is transmitted to a receiving side in RLC PDU format. It is decided to use segmentation and concatenation of RLC SDUs when establishing a radio bearer (hereinafter abbreviated ‘RB’).
A transmitting side transparent mode RLC entity and a receiving side transparent mode RLC entity are configured to transfer data unidirectionally. Therefore, when data transmission is bidirectional, the UE and UTRAN should each be equipped with the transmitting side transparent mode RLC entity and the receiving side transparent mode RLC entity.
When an RLC layer operates in unacknowledged mode, header information is added to an RLC SDU transferred from an upper layer and data is then transmitted to a receiving side in RLC PDU format. If transmission of an RLC PDU fails, retransmission is not supported. Therefore, even if data is lost or a problem occurs during transmission, the receiving side does not request retransmission but rather discards associated data.
In the unacknowledged mode, a transmitting side unacknowledged mode ELC entity transmits data to a receiving side unacknowledged mode RLC entity unidirectionally. Therefore, when data transmission is bidirectional, the UE and UTRAN should each be equipped with the transmitting side unacknowledged mode RLC entity and the receiving side unacknowledged mode RLC entity.
When an RLC layer operates in acknowledged mode, header information is added to an RLC SDU transferred from an upper layer and data is then transmitted to a receiving side in RLC PDU format. If transmission of an RLC PDU fails, retransmission is supported.
Specifically, a transmitting side RLC layer receives status information facilitating a determination of transmission success or failure with regard to a corresponding packet from a receiving side RLC layer and then retransmits an RLC PDU that needs to be retransmitted. In the acknowledged mode, response mode RLC entities, each of which includes a transmitting unit and a receiving unit, exist in both the UE and UTRAN, to enable bi-directional communication.
A radio resource control (RRC) layer located on the lowest part of the third layer is defined in the control plane only and controls the logical channels, the transport channels, and the physical channels with configuration, reconfiguration, and release of radio bearers (RBs). An RB is a service offered by the second layer for the data transfer between the UE 1 and the UTRAN 2. Generally, configuring an RB refers to defining the characteristics of protocol layers and channels necessary for providing a specific service and is to establish respective specific parameters and operational methods for them.
A multimedia broadcast/multicast service (hereinafter, referred to as ‘MBMS’) offers a streaming or background service to a plurality of UEs 1 using a downlink dedicated MBMS bearer service. An MBMS is provided during one session, and data for the MBMS is transmitted to the plurality of UEs 1 via the MBMS bearer service during an ongoing session only. A UE 1 performs activation first for receiving the MBMS to which the UE has subscribed and receives the activated services only.
The UTRAN 2 provides the MBMS bearer service to at least one UE 1 using radio bearers. The radio bearers (RBs) used by the UTRAN 2 include a point-to-point radio bearer and a point-to-multipoint radio bearer.
The point-to-point radio bearer is a bi-directional radio bearer and is configured by a logical channel DTCH (dedicated traffic channel), a transport channel DCH (dedicated channel), and a physical channel DPCH (dedicated physical channel) or a physical channel SCCPCH (secondary common control physical channel). The point-to-multipoint radio bearer is a unidirectional downlink radio bearer and is configured by a logical channel MTCH (MBMS traffic channel), a transport channel FACH (forward access channel), and a physical channel SCPCH. The logical channel MTCH is configured for each MBMS offered to one cell and is used for transmitting user-plane data of a specific MBMS to a plurality of UEs.
As illustrated in FIG. 3, a logical channel MCCH (MBMS control channel) in a conventional system is a point-to-multipoint downlink channel used in transmitting control information associated with the MBMS. The logical channel MCCH is mapped to the transport channel FACH (forward access channel), while the transport channel FACH is mapped to the physical channel SCCPCH (secondary common control physical channel). A cell has only one MCCH.
The UTRAN 2 providing MBMS services transmits MCCH information through the MCCH channel to at least one UE 1. The MCCH information includes notification messages, specifically RRC messages related to the MBMS. For example, the MCCH information may include messages indicating MBMS service information, messages indicating point-to-multipoint radio bearer information or access information indicating that RRC connection for the MBMS is needed.
FIG. 4 is a diagram illustrating how MCCH information is transmitted in a conventional method. FIG. 5 illustrates a conventional method for providing an MBMS.
As illustrated in FIG. 4, the UTRAN 2 providing an MBMS service transmits the MCCH information to a plurality of UEs 1 via the MCCH channel. The MCCH information is periodically transmitted according to a modification period and a repetition period.
The MCCH information is categorized into critical information and non-critical information. The non-critical information can be freely modified each modification period or each repetition period. However, the critical information can be modified only each modification period.
Specifically, the critical information is repeated one time each repetition period. However, the modified critical information can be transmitted only at a start point of the modification period.
If at least two neighbor cells transmit the same information related to an MBMS via an MTCH, a UE receives the MTCH transmitted from the cells by soft combining in order to increase its reception sensitivity. In order to facilitate soft combining, transport blocks from the cells in the same time interval should be identical to each other and a UTRAN should transmit transport blocks for one transport channel only for any given transmission time interval (hereinafter abbreviated TTI).
One transport block is delivered over one transport channel for one TTI. If there are transport blocks to be transmitted, a transmitting side MAC delivers at least one transport block to a physical layer according to a TTI unit. A transmitting side physical layer encodes the transport blocks received from the MAC for transmission to a receiving side physical layer.
In order, to allow the receiving side physical layer to decode the encoded data correctly, as illustrated in FIG. 3, the transmitting side transmits transport format (hereinafter abbreviated ‘TF’) information to the receiving side together with the encoded data. After receiving the TF information from the transmitting side, the receiving side physical layer decodes the received data using the TF in order to reconfigure the transport blocks. The reconfigured transport blocks are transferred to a receiving side MAC according to a TTI unit.
The TF information includes various attributes related to one transport channel. Attributes of the TF information can be classified as semi-static and dynamic. Semi-static attributes are TF information that can be changed slowly by an RRC message. Dynamic attributes are TF information that can be changed fast by TTI or radio frame unit.
A dynamic attribute is transferred via a transport format combination indicator (hereinafter abbreviated ‘TFCI’). The transmitting side transmits the TFCI to the receiving side via a control field of a physical channel.
Typical dynamic attributes of the TF information are transport block size and transport block set size. The transport block size is defined as a set of transport blocks in a TTI.
One transport block is defined as one MAC PDU (protocol data unit), which includes one MAC SDU (service data unit) and one MAC header. At least one transport block can be delivered during one TTI.
The sizes of the transport blocks transmitted for the same TTI are equal. Therefore, a size of the transport block set is a multiple of the size of the transport block.
An RRC establishes a set of size values of transportable transport blocks for one transport channel. A set of the transport blocks of one transport channel is called a transport format set (TFS).
At least two transport channels are mapped to one physical channel for transmission. The RRC establishes a set of combinations of the transport blocks of the transport channels that can be simultaneously transmitted for the same transport section via the corresponding physical channel. A set of the combinations (transport format combinations) of the transport blocks of at least two transport channels mapped to one physical channel is called a TFCS (transport format combination set).
An RNC transmits TFCS information for the physical channel to the transmitting and receiving sides. In order to transmit TFCS information, the RNC transforms the TFCS information into calculated transport format combination (hereinafter abbreviated CTFC) information for transmission to the transmitting and receiving sides rather than directly transmitting the TFCS information.
A connection fame number (herein abbreviated ‘CFN’) is a frame counter used for second layer (Layer2)/transport channel synchronization between a UE and a UTRAN. A CFN value is associated with one TBS and is transmitted via a MAC-L1 SAP.
The CFN is used for common frame reference in Layer2. For example, the CFN can be used for synchronized transport channel reconfiguration.
In MBMS transmission, a UE can increase its reception sensitivity by receiving the same MBMS transport blocks from several neighbor cells providing the same MBMS. In general, transmission of an MBMS transport block can start from a CFN corresponding to a multiplication of the number of frames necessary for transmission of one TTI.
Therefore, if several different MBMS transport blocks are transmitted by being mapped to one channel, transmission of MBMS transport blocks may not be possible within a required time to enable soft combining. In order to solve this problem, MBMS service transmission is delayed, thereby wasting channel resources.
Therefore, it is necessary to initiate transmission of MBMS transport blocks from different timing points as well as the CFN corresponding to the multiplication of frames required for the transmission of one TTI. However, in the conventional method, a UE is unable to determine information related to a transmission start timing point of an MBMS transport block, thereby reducing efficiency of soft combining.
Therefore, there is a need for a method and apparatus for transmitting/receiving multimedia broadcast/multicast service (MBMS) transport blocks that enables a UE to determine information related to a transmission start timing point of an MBMS transport block, thereby increasing efficiency of soft combining. The present invention addresses these and other needs.